The present invention relates to an exclusion circuit employed in a key telephone system comprising a number of telephone sets, each accommodating a plurality of lines, and the exclusion circuit having functions including the function of originating and answering calls over the telephone lines, the function of selecting and holding a line, and the function of permitting intercommunications between telephone sets.
A key telephone system comprises a number of key telephone sets connected to each other, in a multiple form, by way of line buttons. In such system, speech between two telephone sets can readily be tapped by another telephone set when connected to the associated line. It has therefore been necessary to provide some means capable of preventing speech between two telephone sets from being tapped through the associated line of speech channel.
This privacy-protective function must be workable in such manner that a telephone set which is to initiate a call may be connected to the desired line only when such line is idle, and the line engaged thereby is held thereafter and accessed by another telephone set.
Generally, a semiconductor device with switching and holding characteristics, such as thyristor, is used in the prior art exclusion circuit of a key telephone set. This semiconductor device must be used with a protective circuit and a misoperation preventive circuit, with the result that the circuitry is inevitably complicated. Furthermore, the semiconductor device used is expensive.